Effect of CO2 diluent on fuel versus oxidizer side of spherical diffusion flames in microgravity

The present study experimentally investigates fundamental characteristics of diffusion flames and the importance of gas radiation. In particular, transient measurements of flame growth, temperature, and radiation intensity of an expanding spherical diffusion flame in microgravity are used to investigate the effects of various diluents (N2, CO2) with different radiative properties. Two systems were studied: diluents on the oxidizer side of the reaction zone and diluents on the fuel side. Experiments were conducted at atmospheric pressure using a porous spherical burner aerodynamically supporting an ethylene diffusion flame in an oxidizing atmosphere. A color CCD camera was used to obtain the flame growth rate. Temperature field was measured by an array of thermocouples that were corrected for radiation, conduction through the leads and time constant of the bead. Radiation intensity was measured by an array of photodetectors sensitive to selected spectral regions. Experimental results show that Lewis number of the oxidizer (LeO) does not have a dominant effect on the flame growth and the effect of the Lewis number of the fuel (LeF) is unclear from the experimental results. Despite controlling qCP, the energy storage component of the reactant mixtures, influence of CO2 in the flame region reduced the flame temperature and the soot formation due to increased energy transfer. CO2 present in the oxidizer increased the volume-averaged temperature and lowered the flame emission indicating both an increase in radiation heat transfer and radiation reabsorption. The presence of CO2 in the fuel decreased the volume-averaged temperature significantly. Overall, CO2 diluent affected the two systems by different mechanisms. On the oxidizer side, radiation reabsorption played a role in strengthening the flame, while on the fuel side, enhanced CO2 increased radiative heat losses without exhibition of radiation reabsorption. 2006 Published by Elsevier Inc. on behalf of The Combustion Institute.